Initial Modelling of a Novel Actuator for the Entrainment of Fast MHD Modes by Slow 3D Field Coils

Simulations using a simplified filament model are used to assess the net torque that can be induced onto quickly-rotating magnetic islands by slowly changing applied 3d magnetic fields. Previous work has demonstrated that rotating 3d fields can be used to entrain MHD modes to the frequency of the applied field. However, the frequency of the applied fields is constrained by the slew-rate of the power supplies, the presence of conducting structures, and the reactance of the 3d coils themselves. This work finds that regular impulses of 3d field coils may allow some circumvention of these limitations. Similar to pushes on a swing set, this novel technique relies on the fact that the frequency of the driver does not necessarily have to be tied to the frequency of the driven system, as long as there is sufficient system inertia and several key conditions are met. Presented here are the concept and mathematical basis for the technique, some initial modelling of example pulses and the net imparted torque, as well as conceptual examples of how such a technique could be used, with special focuses on application to DIII-D and extension to ITER.